Correct spelling

31st

Correct spelling, explanation: this form is correct, because the last number pronounced here is first which ends with st. Similarly 55th is pronounced fifty-fifth and 63rdsixty-third. Thirty-first has got st at the end, which is added to a number form as an abbreviation. This is the reason why 31st is the correct form, and not 31th.

Definition of 31st:
numeral, ordinal number between 30th and 32nd
He got a trip to Madrid for his 31st birthday.


Incorrect spelling

31th

Incorrect spelling, explanation: writing 31th is incorrect, because in the full-word form this number is pronounced as thirsty-first so at the end there is st which should become an abbreviation at the end of number form. This way we know that the correct form is 31st.

Incorrect spelling

31nd

Incorrect spelling

Incorrect spelling

31rd

Incorrect spelling